Permselective Membrane Research Articles

Oxygen Permeability of Fully Conjugated Polymers Consisting of (Di)ethynylene and Phenylene Groups Synthesized by Glaser or Sonogashira Coupling Polycondensation

Fully conjugated polymers are candidates as rigid polymers for good oxygen permselective membrane materials. In this study, we synthesized a series of such polymers with a rigid backbone consisting of (di)ethynylene and phenylene groups and alkyl side groups by Glaser or Sonogashira coupling polycondensation. Some of these polymers gave self-supporting membranes and their oxygen permselectivities were influenced by the alkyl groups. Oxygen permselectivities of polymers containing diynes in the main chain were determined for the first time.

Vacancy Engineering for High-Efficiency Nanofluidic Osmotic Energy Generation.

Two-dimensional (2D) nanofluidic membranes have shown great promise in harvesting osmotic energy from the salinity difference between seawater and fresh water. However, the output power densities are strongly hampered by insufficient membrane permselectivity. Herein, we demonstrate that vacancy engineering is an effective strategy to enhance the permselectivity of 2D nanofluidic membranes to achieve high-efficiency osmotic energy generation. Phosphorus vacancies were facilely created on NbOPO4 (NbP) nanosheets, which remarkably enlarged their negative surface charge. As verified by both experimental and theoretical investigations, the vacancy-introduced NbP (V-NbP) exhibited fast transmembrane ion migration and high ionic selectivity originating from the improved electrostatic affinity of cations. When applied in a natural river water|seawater osmotic power generator, the macroscopic-scale V-NbP membrane delivered a record-high power density of 10.7 W m-2, far exceeding the commercial benchmark of 5.0 W m-2. This work endows the remarkable potential of vacancy engineering for 2D materials in nanofluidic energy devices.

High hydrogen permselective carbon molecular sieve membrane and its structural formation mechanism

A novel hydroxyl-containing polyetherimide (named BAHPPF-ODPA) with highly aromatic backbone and contorted chain conformation was designed and synthesized as the precursor to fabricate carbon molecular sieve membranes (CMSMs) for H2/N2 separation. The structural evolution from polymeric precursor to carbon during pyrolysis was investigated using characterization techniques combined with pyrolysis simulation. And the relationship between membrane structure formation and their separation performance was explored here. Results indicate that the generation and evolution of pyrolysis products including formed fragments and gases, which depend on the precursor chemistry and pyrolysis conditions, exhibited a significant influence on microstructure and performance of derived CMSMs. Designing precursors with an aromatic backbone and contorted structure and precisely tuning pyrolysis temperature allows the fabrication of CMSMs with controllable micropore structure and excellent H2 separation performance. The BAHPPF-ODPA derived CMSM pyrolyzed at 600 °C exhibited the high H2 permeability of about 3500 Barrer and the superior H2/N2 selectivity of more than 400 were obtained after 750 h of long-term mixed gas (75:25 mixture of H2/N2) testing, showing promising application prospects. The experimental and simulation results obtained here would be beneficial to understand the CMS formation and reasonable to design the precursor molecular structure for fabrication of high-performance CMSMs.

Fluorinated Covalent Organic Framework-Based Nanofluidic Interface for Robust Lithium-Sulfur Batteries.

To realize the practical application of lithium-sulfur (Li-S) batteries, there is a need to inhibit uncontrolled Li deposition by facilitating Li-ion migration, and suppress the irreversible consumption of cathodes by preventing polysulfide shuttling. However, a permselective artifical membrane or interlayer which features fast ion transport but low polysulfide crossover is elusive. Here, we report the design and synthesis of a fluorinated covalent organic framework (4F-COF)-based membrane with a high permselectivity and increased battery lifespan. Combining density functional theory calculation, molecular dynamic simulation, and in situ Raman analysis, we demonstrate that fluorinated COF eliminates polysulfides shutting and dendritic lithium formation. Consequently, Li symmetrical cells demonstrate Li plating/stripping behaviors for 2000 h under 1 mA cm-2. More importantly, Li-S batteries based on the 4F-COF/PP separator achieve cycling retention of 82.3% over 1000 cycles at 2 C, rate performance of 568.0 mA h g-1 at 10 C, and an areal capacity of 7.60 mA h cm-2 with a high sulfur loading (∼9 mg cm-2). This work demonstrates that functionalizing nanochannels in COFs can impart permselectivity for energy storage applications.

Changes in permselectivity of radiation-grafted anion-exchange membranes with different cationic headgroup chemistries are primarily due to water content differences

For anion-exchange membranes, different cationic chemistries change the balance between permselectivity and conductivity, due to changes in water content.

Regulating interfacial polymerization via a multi-functional calcium carbonate based interlayer for a highly permselective nanofiltration membrane

The CaCO3 interlayer was grown in situ on PK membrane through mineralization process, which plays a multi-functional role in the process of regulating IP and facilitating the formation of a thinner selective layer with rough and loose structure.

Effect of [Na+]/[Li+] concentration ratios in brines on lithium carbonate production through membrane electrolysis.

Lithium is a fundamental raw material for the production of rechargeable batteries. The technology currently in use for lithium salts recovery from continental brines entails the evaporation of huge water volumes in desert environments. It also requires that the native brines reside for not less than a year in open air ponds, and is only applicable to selected compositions, not allowing its application to more diluted brines such as those geothermally sourced or waters produced from the oil industry. We have proposed an alternative technology based on membrane electrolysis. In three consecutive water electrolyzers, fitted alternately with anion and cation permselective membranes, we have shown, at proof-of-concept level, that it is possible to sequentially recover lithium carbonate and several by-products, including magnesium and calcium hydroxide, sodium bicarbonate, H2 and HCl. The big challenge is to bring this technology closer to practical implementation. Thus, the issue is how to apply relatively well-known electrochemical technology principles to large volumes and to a highly complex and saline broth. We have studied the application of this new methodology to ternary mixtures (NaCl, LiCl and KCl) with constant LiCl and KCl composition and increasing NaCl content. Results showed very similar behaviour for systems containing [Na+]/[Li+] concentration ratios ranging from 1.24 to 4.80. The voltage developed between the anode and cathode is almost the same in all systems at roughly 3.5 V when a constant current density of 50 A m-2 is applied. The three monovalent cations migrate with different rates across the cation exchange membrane, with Li+ being the most sluggish and thus crystallization of Li2CO3 only occurs close to completion of the electrolysis. The dimensionless concentration profiles are almost indistinguishable despite the changes in total salinity. The solids crystallized from different feeds showed higher Na+ and K+ contents as the initial Na+ concentration was increased. However, solids with over 99.9% purity in Li2CO3 could be obtained after a simple re-suspension treatment in hot water. The electrochemical energy consumption greatly increases with higher Na+ concentrations, and the amount of fresh water that can be recovered is diminished.

Breakthroughs on tailoring membrane materials for ethanol recovery by pervaporation

• Mixed matrix membranes (MMMs) with well-designed membrane structures may provide both high permeability and membrane selectivity to break trade-off effect due to the better control over physi-/chemical micro-environment and cavity/pore size as well as size distribution. • Fillers should be orderly arranged in polymer matrix and form continuous transport channels for ethanol to full play the synergistic effect of polymer and fillers. • Building firm bonding interactions between filler and polymer is one of the most promising methods to strengthen the PV performance of MMMs. • The development of bioinspired materials and structures provides enlightening ideas for the construction of ethanol perm-selective membranes. Bioethanol, as a clean and renewable fuel, has gained increasing attention due to its major environmental benefits. Pervaporation (PV) is a promising and competitive technique for the recovery of ethanol from bioethanol fermentation systems due to the advantages of environmental friendliness, low energy consumption and easy coupling with fermentation process. The main challenge for the industrial application of ethanol perm-selective membranes is to break the trade-off effect between permeability and selectivity. As membrane is the heart of the pervaporation separation process, this article attempts to provide a comprehensive survey on the breakthroughs of ethanol perm-selective PV membranes from the perspectives of tailoring membrane materials to enhance PV separation performance. The research and development of polymeric and organic/inorganic hybrid membranes are reviewed to explore the fundamental structure-property-performance relationships. It is found that mixed matrix membranes with well-designed membrane structures offer the hope of better control overphysi-/chemical micro-environment and cavity/pore size as well as size distribution, which may provide both high permeability and membrane selectivity to break the trade-off effect. The tentative perspective on the possible future directions of ethanol perm-selective membranes is also briefly discussed, which may provide some insights in developing a new generation of high-performance PV membranes for ethanol recovery.

Low-temperature ozone treatment for p-xylene perm-selective MFI type zeolite membranes: Unprecedented revelation of performance-negating cracks larger than 10 nm in polycrystalline membrane structures

We report that low-temperature ozone calcination allowed MFI type polycrystalline zeolite membranes to maximize their p-/o-xylene separation factor (as high as ca. 2000) by suppressing defect formation. Conventional high-temperature calcination and rapid thermal processing, which provide poor and marked p-/o-xylene separation abilities, respectively, were used for comparison. The corresponding defect structures were quantitatively analyzed by image processing of fluorescence confocal optical microscopy images combined with membrane permeation modeling, revealing the main defects (grain boundary defects and cracks) and their tortuosity, porosity, and size. To the best of our knowledge, we, for the first time, demonstrated that in contrast to common belief, the minor portion of wider cracks instead of the major portion of narrower grain boundary defects determined the final permeation rates. Specifically, the MFI membrane prepared by high-temperature calcination contained many grain boundary defects (narrow; ca. 1 nm) and few cracks (wide; ca. 20 nm) that accounted for ca. 0.1% and 99.8%, respectively, of the slowly permeating o-xylene molar flux. In contrast, the ozone-treated MFI membranes, which only possessed grain boundary defects, achieved the high p-/o-xylene separation performance, underlining the need for the selective reduction in the number of wider cracks rather than ubiquitous, narrow grain boundary defects.

Hetero-metal solution to construct high permselective zeolite membranes

Zeolite membranes possess great application prospects in purifying chemical substances, however, the current synthetic conditions are insufficient to eliminate defects, restricting the industrial applications of zeolite membranes. In this study, compact X-type zeolite membranes were synthesized in hetero-alkali-metal cation synthesis solutions via in-situ heating method. The obtained X-type zeolite membrane possessed a methanol/methyl tert-butyl ether (MTBE) separation factor greater than 10,000 at a permeation flux of 1.3 kg m−2 h−1. A possible mechanism for the influence of hetero alkali meatal cations on the formation of zeolite membranes was proposed, which involved structural induction and regulation of the rates of crystal nucleation and growth. The hetero-alkali-metal cations entering the zeolite crystal caused distortion of the crystal structure, which promoted transformation of the zeolite crystal structure and accelerated crystal growth. Meanwhile, the rates of crystal nucleation and growth on the surface of the support were regulated by the cooperative action of the hetero-alkali-metal cations and Na+ in the synthesis solution through their solubility for the aluminosilicate gel, leading to the formation of compact zeolite membranes. This method controlled the zeolite crystal structure and compactness of the zeolite membrane by simply adding hetero-alkali-metal cations to the synthesis solution, providing a new idea for designing defect-free membranes with high permselectivity.

Overcoming the Dilemma of Permeability and Stability of Polymersomes through Traceless Cross-Linking.

In nature, cells are highly compartmentalized into many organelles that are well separated from the rest of the cellular space by unique membrane structures, which are of crucial importance to allow cells to perform various physiological functions in such a small and crowded space. Learning from the ubiquitous membrane structures of cells and organelles has continuously inspired the development of artificial self-assembled nanostructures, with lipid vesicles (liposomes) and polymer vesicles (polymersomes) being the most representative examples. Similar to the membrane-bound structures of cells and organelles, both liposomes and polymersomes contain an aqueous interior enclosed by a bilayer membrane. Therefore, liposomes and polymersomes have been extensively investigated to mimic the fundamental structures and functions of living cells. For example, liposomes and polymersomes have been successfully engineered as nanocarriers, smart nanoreactors, artificial organelles, and so on. Notably, living cells can exchange both energy and materials with surrounding environments, benefiting from the selective permeability of lipid membranes. The permselectivity of cell membranes is thus an essential attribute of living organisms. Compared to liposomes, polymersomes have increased structural stability but low membrane permeability. Indeed, polymersomes are almost impermeable to small molecules, ions, and even water molecules. To improve the permeability of polymersomes, much effort has been devoted to the incorporation of channel proteins, the coassembly of oppositely charged block copolymers (BCPs), the development of stimuli-responsive BCPs, and so on. Despite great achievements, these approaches generally lead to decreased stability of polymersomes and, sometimes, polymersome disintegration. In this Account, we discuss our recent efforts to reconcile the stability and permeability of polymersomes via a traceless cross-linking approach. Although cross-linking reactions within bilayer membranes generally lead to decreased permeability, the traceless cross-linking approach can concurrently improve the stability and permeability of polymersomes. Specifically, stimuli-responsive polymersomes undergo either covalent cross-linking or noncovalent cross-linking reactions under specific stimuli to increase bilayer stability, while the cross-linking processes can concurrently permeabilize polymersome bilayers through cross-linking-driven hydrophobic-to-hydrophilic transitions. Notably, unlike conventional cross-linking processes requiring additional cross-linkers, the traceless cross-linking process does not involve extra cross-linking agents but takes full advantage of the in situ generated active moieties. By taking advantage of the simultaneous modulation of the stability and permeability of polymersomes via traceless cross-linking, these polymersomes can be further engineered as smart nanocarriers and nanoreactors. The robustness and generality of this approach have been validated by both extracellular and intracellular stimuli such as light irradiation, glutathione, and hydrogen peroxide. Moreover, many functional groups such as fluorescent dyes and contrast agents can be integrated into this versatile platform as well, enabling the construction of theranostic nanovectors capable of responding to pathological microenvironments. This Account provides a new approach to regulating the permeability of polymersomes while maintaining their structural stability.

Orientation of the microstructure of free-standing carbon hollow fiber membranes modulated by carbonization condition triggers aging-resistant properties

The aim of this study is to determine the optimal carbonization conditions for carbon hollow fiber membranes for hydrogen purification. To this end, it is a prerequisite to obtain the best permselectivity of the resultant membrane. Three different carbonization atmospheres (vacuum, helium flow, and argon flow) were examined, while their effects on the changes in the microstructure and gas permeation properties of the as-synthesized carbon membranes were elucidated using a pure gas permeation test, coupled with an array of characterizations, including Raman spectroscopy and XPS. Moreover, the issue of vacuum degree was addressed to maintain the best reproducibility of the carbon membrane. The results revealed that CHFM-HV exhibited the best separation capability for H2/CH4 of 516.90, thereby indicating distinct hydrogen purification characteristics. Such good performance was driven by the attractive synergism of the ordered microstructure and larger pore volume. Notably, the recovery of permselectivity for CHFM-HV reached 90% after four months, thus suggesting its aging resistance. Hence, the demonstrated approach is efficient for altering the microstructure of the carbon membrane, thereby strengthening the separation performance and long-term stability. Overall, it is suitable for broad applications of diverse polymer precursors and prolonged the lifespan of the as-prepared carbon membrane.

Oxidation resistances of polyamide nanofiltration membranes to hydroxyl and sulfate radicals

The oxidation compatibility with polyamide membranes is an important feature of nanofiltration (NF) technology in practical water treatment processes. In this study, we systematically investigated the oxidation resistance of fully aromatic (NF90) and semi-aromatic (NF270) polyamide NF membranes to reactive oxidative species including SO4•- and HO• in terms of membrane surface properties and filtration performance. SO4•− and HO• exhibit different oxidative behaviors in NF membranes with different polyamide structures. The HO•-dominated oxidation significantly damaged the membrane integrity of NF90 and NF270, with notably varied surface morphology, hydrophilicity, and charge. Further analysis of the chemical property on membrane surfaces revealed that the SO4•−-dominated oxidation was more reactive to the N moieties (N–H or N–C) of the NF90 membrane, whereas HO• unselectively reacted with polyamides. The radicals also impact the permselectivity (water permeability and salt rejection) of the NF90 and NF270 membranes in different ways. The NF90 membrane after SO4•−-exposure exhibited higher water permeability and slightly decreased salt rejection, whereas the permselectivity of the NF270 membrane was more significantly affected by HO•-exposure. To further reveal the underlying oxidation mechanisms, polyamide monomers were selected to react with the peroxodisulfate solutions. The results demonstrated that a fully aromatic polyamide membrane was more resistant to oxidative radicals than a semi-aromatic polyamide membrane. This study will facilitate the development of novel antioxidant membranes and provide information for selecting appropriate membrane materials in specific water treatment processes.

Nanomaterials with oxygen mobility for catalysts of biofuels transformation into syngas, SOFC and oxygen/hydrogen separation membranes: Design and performance

Modern hydrogen energy technologies include generation of syngas and pure hydrogen from biofuels to be used for green energy production in solid oxide fuel cells (SOFC). To be economically feasible, these processes require design of stable, efficient and inexpensive catalysts, permselective membranes for hydrogen and oxygen separation as well as SOFC cathode and anode materials. In all cases, such materials should possess a high oxygen mobility. This review presents results of our research devoted to design of materials based on complex oxides with fluorite, perovskite, and spinel structures used in these applications. The key aspect is characterization of their oxygen mobility by unique method of temperature –programmed oxygen heteroexchange in flow reactors and finding its dependence on their composition, real structure/microstructure and surface properties affected by methods of synthesis and elucidated with the help of modern structural, spectroscopic and kinetic methods. Optimized materials were shown to provide stable and efficient performance in catalytic reactors, membranes and solid oxide fuels cells.

Donnan dialysis for phosphate recovery from diverted urine

There is a critical need to shift from existing linear phosphorous management practices to a more sustainable circular P economy. Closing the nutrient loop can reduce our reliance on phosphate mining, which has well-documented environmental impacts, while simultaneously alleviating P pollution of aquatic environments from wastewater discharges that are not completely treated. The high orthophosphate, HxPO4(3−x)−, content in source-separated urine offers propitious opportunities for P recovery. This study examines the use of Donnan dialysis (DD), an ion-exchange membrane-based process, for the recovery of orthophosphates from fresh and hydrolyzed urine matrixes. H2PO4− transport against an orthophosphate concentration gradient was demonstrated and orthophosphate recovery yields up to 93% were achieved. By adopting higher feed to receiver volume ratios, DD enriched orthophosphate in the product stream as high as ≈2.5 × the initial urine feed concentration. However, flux, selectivity, and yield of orthophosphate recovery were detrimentally impacted by the presence of SO42− and Cl− in fresh urine, and the large amount of HCO3− rendered hydrolyzed urine practically unsuitable for P recovery using DD. The detrimental effects of sulfate ions can be mitigated by utilizing a monovalent ion permselective membrane, improving selectivity for H2PO4− transport over SO42− by 3.1 × relative to DD with a conventional membrane; but the enhancement was at the expense of reduced orthophosphate flux. Critically, widely available and low-cost/waste resources with sufficiently high Cl− content, such as seawater and waste water softening regenerant rinse, can be employed to improve the economic viability of orthophosphate recovery. This study shows the promising potential of DD for P recovery and enrichment from source-separated urine.

Soluble Imide-Bridged Polypentamethyltrisiloxane (IBPPMS) with Rationally Designed Ladder-like Structure for O2/N2 Permselectivity

Oxygen/nitrogen (O2/N2) permselective membranes are widely used in both industrial and medical applications. Polysiloxane is one of the most important membrane materials with high oxygen permeability, but its poor capacities for gas separation and membrane formation limit its practical use. In this study, a soluble imide-bridged polypentamethyltrisiloxane (IBPPMS) with a high molecular weight (Mn = 8.91 × 104) was designed and synthesized via a hydrosilylation reaction between polypentamethyltrisiloxane (PPMS) and N,N′-bis(3-allyl)pyromellitic diimide (DAI) in NMP under Karstedt’s catalysis. The ladder-like structure of sample was analyzed and determined through infrared spectroscopy (IR), nuclear magnetic resonance spectroscopy (1H NMR and 29Si NMR), and gel permeation chromatography (GPC). The IBPPMS membrane displays excellent thermal resistance. Furthermore, its O2 permeation performance achieves 47.1 barrer, and its separation factor reaches 7.11 with a feeding proportion of 50% each for O2/N2, exceeding the 2008 Roberson upper bound. Thus, it appears that membranes can achieve high selectivity and permeability as well as exhibit desirable thermal properties and sufficient stability when separating numerous industrial significant gas mixtures. The discovery of trapezoidal structural backbone of this polymer materials substantially affects their separation properties and sustained performances.

Numerical simulation of ion transport across monovalent ion perm-selective membranes

The separation of specific ions is common in chemical processes, including lithium mining, hydrometallurgy, and biochemical engineering. The monovalent cation perm-selective membrane is a vital material for ion classification. Although many advanced membranes with sufficient permeability and selectivity were developed, the systematical relationships between membrane function and ion transport character are still unclear. In this work, we successfully developed a two-dimensional model based on Navier-Stokes and Nernst-Planck-Poisson equations asserted through numerical simulation. The impact of modified layer's thickness, fixed charge concentration, as well as base membrane fixed charge density on ion flux and selectivity were thoroughly investigated. The combined contribution of membrane properties on the trade-off between ion selectivity and permeability was thus established using this innovative simulation and modeling strategy. This is the first systematic research to provide a substantial reference for the development of advanced cation perm-selective membranes for monovalent ions isolation.

Crystallization-controlled defect minimization of a ZIF-67 membrane for the robust separation of propylene and propane

Translating rigid structural ZIF-67 into a highly permselective membrane for the robust separation of C3H6 and C3H8 is desirable but remains challenging. The way to this goal requires the precise control of grain boundary structure to minimize non-intrinsic transport pathways. This study reports a time-saving and reliable rapid thermal conversion strategy to prepare a high-quality ZIF-67 membrane. In this case, the ligand-to-metal ratio determines the quality of the grain boundary structure, which can be rationally modulated to achieve a well-constructed membrane configuration. The resultant ZIF-67 membrane harvested an impressive C3H6/C3H8 selectivity of over 120 and exhibited exceptional thermal stability as well as adaptability to the C3H6/C3H8 feeds with different compositions. Because of the suppressed inter-grain defects, the ZIF-67 membrane delivered improved C3H6/C3H8 separation performance at a feed pressure range of 1–7 bar. Moreover, the membrane demonstrated outstanding long-term durability over a 32-day measurement. By reducing the synthetic solution concentration, the membrane permeance of C3H6 can be significantly increased from 1.0 × 10−8 mol m−2 s−1 Pa−1 to 4.8 × 10−8 mol m−2 s−1 Pa−1 without compromising the selectivity. It made the ZIF-67 membrane one of the top-performing candidates for robust C3H6/C3H8 separation.

From islet of Langerhans transplantation to the bioartificial pancreas

Type 1 diabetes is a disease resulting from autoimmune destruction of the insulin-producing beta cells in the pancreas. When type 1 diabetes develops into severe secondary complications, in particular end-stage nephropathy, or life-threatening severe hypoglycemia, the best therapeutic approach is pancreas transplantation, or more recently transplantation of the pancreatic islets of Langerhans. Islet transplantation is a cell therapy procedure, that is minimally invasive and has a low morbidity, but does not display the same rate of functional success as the more invasive pancreas transplantation because of suboptimal engraftment and survival. Another issue is that pancreas or islet transplantation (collectively known as beta cell replacement therapy) is limited by the shortage of organ donors and by the need for lifelong immunosuppression to prevent immune rejection and recurrence of autoimmunity.A bioartificial pancreas is a construct made of functional, insulin-producing tissue, embedded in an anti-inflammatory, immunomodulatory microenvironment and encapsulated in a perm-selective membrane allowing glucose sensing and insulin release, but isolating from attacks by cells of the immune system. A successful bioartificial pancreas would address the issues of engraftment, survival and rejection. Inclusion of unlimited sources of insulin-producing cells, such as xenogeneic porcine islets or stem cell-derived beta cells would further solve the problem of organ shortage.This article reviews the current status of clinical islet transplantation, the strategies aiming at developing a bioartificial pancreas, the clinical trials conducted in the field and the perspectives for further progress.

Permselective graphene-based membranes and their applications in seawater desalination

Permselective graphene-based membranes and their applications in seawater desalination